Triacylglycerol (TAG), an important molecule for eukaryotic fuel storage, is synthesized by two major pathways, the glycerol 3-phosphate pathway and the monacylglycerol pathway. The glycerol 3-phosphate pathway is present in all tissues whereas the monoacylglycerol pathway is restricted to the enterocytes of the small intestine (Bell, R. M. and R. A. Coleman, Annu. Rev. Biochem. (1980). 49:459-87). The monoacylglyerol pathway is believed to be critical for the packaging of dietary fat into chylomicron lipoprotein particles (Levy, E., Can. J. Physiol. Pharmacol. (1992) 70(4):413-9).
Acyl coenzyme A:monoacylglycerol acyltransferase (“MGAT”) (EC 2.3.1.22) is an enzyme best known for its role in initiating the first step for the monoacylglycerol pathway (Lehner, R. and A. Kuksis, Prog. Lipid Res. (1996) 35(2):169-201). In order for insoluble dietary fat such as TAG to be absorbed by the intestine, dietary fat molecules must first be digested by pancreatic lipase into soluble free fatty acids and 2-monoacylglycerol. These products are quickly absorbed into enterocytes, within minutes of their appearance in the lumen of the small intestine, MGAT uses these molecules as substrates to form diacylglycerol (DAG). DAG is further acylated by diacylglycerol acyltransferase (DGAT) to re-form TAG. The newly formed TAG molecules are then packaged with other complex lipids such as cholesterol ester, phospholipids and small amount of protein to form round lipoprotein particles called chylomicrons. Chylomicrons, 90% of which are comprised of TAG, are quickly secreted into the lymph where they served as energy supplies for the body (Lehner, R. and A. Kuksis, supra).
Similar to other neutral lipid synthesis proteins, MGAT is an intrinsic membrane protein which to date has not been purified to homogeneity from any sources. The molecular identity of the gene encoding the intestinal MGAT has been elusive. The first cDNA clone shown to possess MGAT enzyme activity, designated MGAT1, was identified by Yen et al. (Yen, C. L., et al., Proc. Natl. Acad. Sci. USA (2002) 99(13):8512-7). However, MGAT1 is expressed in stomach, kidney, white adipose and brown adipose tissue, but not in small intestine. Therefore, MGAT1 cannot account for the high intestinal MGAT enzyme activity that is important for the physiology of fat absorption (Yen, C. L., et al., supra).
It is known that drugs that inhibit the absorption of dietary fat can be efficacious for obesity treatment (Zhi, J., et al. (1995) J. Clin. Pharmacol., 35(11):1103-8; Lucas, K. H. and B. Kaplan-Machlis (2001) Ann. Pharmacother., 35(3):314-28). However the mechanism for such drugs includes the inhibition of pancreatic lipase, which leads to the undesirable side effect of faecal leakage (Kolanowski, J. (1999) Drug Saf., 20(2):119-31). An alternative approach is to allow for the digestion of fat to fatty acids in the gut, and then block the uptake or the packaging of fatty acids into chylomicron particles.
As intestinal MGAT is a critical enzyme for this pathway, there is a continuing need to identify genes responsible for MGAT intestinal enzyme activity, and therefore responsible for the absorption of dietary fat. The modulation of such genes will provide methods for the treatment of obesity. The present invention is directed to such a need.